An Information Retrieval Approach for Source Code Plagiarism Detection

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1 -2014: An Information Retrieval Approach for Source Code Plagiarism Detection Debasis Ganguly, Gareth J. F. Jones CNGL: Centre for Global Intelligent Content School of Computing, Dublin City University Dublin 9, Ireland. December 6, 2014

2 Overview Research Challenges Related Work Introduction Question answering forums and programming related blogs have made source code widely available to be read, copied and modified. Tendency to copy-paste code. Can result in IP violation. Research Challenge: Automatically detect cases of software plagiarism. SOCO 2014: Dataset to evaluate plagiarism detection performance.

3 Overview Research Challenges Related Work Overview of our Approach We undertake an information retrieval (IR) approach to solve this problem. Advantages: No need for exhaustive pairwise comparison between documents. Faster than pair-wise exhaustive approaches. An ad-hoc query mechanism: a ranked list of candidate documents can be retrieved for a suspected document.

4 Overview Research Challenges Related Work Research Challenges Q1: Does a bag-of-words model (as used in standard IR) suffice or should the source code structure be used in some way to extract more meaningful pieces of information? Q2: How to index the source code documents so that a retrieval model can best make use of the indexed terms to retrieve relevant (plagiarized) documents at top ranks? Q3: How to represent a source code document as a pseudo-query, i.e. what are the most likely representative terms in a source code document?

5 Overview Research Challenges Related Work Limitations of Existing Methods Near Duplicate Document Detection by Shingling. Only a part of the source code is copied for reuse. Occurrences of exact duplicates at the level of whole documents is rare. Jacard coefficient of the shingles is expected to be low. Bag-of-words Model False high similarity values between non-plagiarized documents pairs. Because of the use of similar programming language specific constructs and keywords. Example: Programs tend to use a frequent set of variable names, specially for looping constructs, e.g. i, j, k etc.

6 Overview Research Challenges Related Work Limitations of Existing Methods (Contd.) Exhaustive Pair-wise Similarity Most popular approach that works well in practice. Does not scale well for large collections. Often tested on a small collection, e.g. (Chae et. al., 2013) uses a collection of 56 programs for evaluation.

7 Document Representation Retrieval Source Code Parsing Different parts of source codes need to be matched differently. Example: A program which defines a class HelloWorld vs. a program which defines a string literal HelloWorld. Parse the source code to build an annotated syntax tree (AST). Extract terms from specific nodes of the AST.

8 Document Representation Retrieval Source Code Parsing (Contd.) Table: Annotated Syntax Tree nodes of a Java program from which terms are extracted during indexing. Field Name Field Description Classes The names of the classes used in a Java source Method calls The method names and actual parameter names and types String literals Values of the string constants Arrays Names of arrays and dimensions Method definitions Names of methods and formal parameter names and types Assignment statements Variable names and types Package imports Names of imported packages Comments

9 Document Representation Retrieval Pseudo-Queries from Source Codes It is not reasonable to use whole source code documents as a pseudo-queries: A part of the source code is typically copy-pasted. Extract a pre-set number of terms from each field of a document by a term scoring function. We use the LM term scoring function. tf (t, f, d) (t) LM(t, f, d) = λ + (1 λ)cf len(f, d) cs (1)

10 Document Representation Retrieval Plagiarized Set from Ranked List Thresholding on the relative drops in similarity values of the ranked list. Plag(Q) = {D i : sim(q, D i) sim(q, D i 1 ) ɛ} (2) sim(q, D i 1 )

11 Baselines Run-1: Standard LM retrieval: Flat bag-of-words representation. Source codes treated as non-structured text documents. Index does not constitute of separate fields. Run-2: Terms extracted from the selected nodes of the AST. No separate fields are used to store them. Other approaches explored (not submitted in the SOCO task): Using different number of terms while constructing the pseudo-queries. Unigram and bi-gram (word level) indexing.

12 Training Set Results Parametes Metric AST Fields #Qry terms n-gram Precision Recall F-score no no all no no yes no all yes no yes yes all yes yes no no all no no yes no all yes no yes yes all yes yes

13 Test Set (Official SOCO) Results Run Parametes Metric Parse Fields #terms n-gram Prec. Rcll F-sc dcu-run1 no no dcu-run2 yes no dcu-run3 yes yes Test set results somewhat different from the training set ones. A flat index constituted from the AST terms produces very good results (which was not the case for the training set). Flat indexing with no parsing worse precision (and hence F-score) in comparison to the parsing based approaches. Surprisingly, field based LM does not turn out to be more effective than the standard bag-of-words LM.

14 Conclusions Q1: Program structure is important for determining source code plagiarism. (Both the development set and the official results empirically confirm this). Q2: Field based retrieval model improves results further? Inconclusive from the results of the development and the test sets. Q3: Results show that an LM based term selection method of selecting representative terms works significantly better than using all terms for pseudo-query formulation.